One of such hydraulic driven punchers is a reciprocal, automatic-return type puncher. An example of the reciprocal, automatic-return puncher is schematically illustrated in FIG. 7.
As shown, there is a punch 32 attached to the lower end of a ram 31. An upper limit switch 33 and a lower limit switch 34 are provided on both sides of the upper part of the ram 31. A hydraulic pump 36 is arranged for delivering a flow of hydraulic oil via a directional control (solenoid operated) valve 37 (abbreviated to directional value 37 hereinafter) to a cylinder 35. This allows the ram 31 to move upward and downward in reciprocal action. A controller 41 is provided responsive to command signals from a start (downward operation) switch 42 and an upward operation switch 43 and detection signals from the upper 33 and the lower limit switch 34 for controlling the actions of the hydraulic pump 36 and the directional valve 37.
It is now assumed that the ram 31 stays at the upper end of its movement and the upper 33 and the lower limit switch 34 remain pressed down with its contacts closed. When the start switch 42 is turned on, an upper chamber 35a of the cylinder 35 is loaded with the hydraulic oil while a lower-chamber 35b is exhausted. As the ram 31 starts moving downward, the contact of the upper limit switch 33 is opened. When the ram 31 arrives at the lower limit of its movement, the contact of the lower limit switch 34 is opened and the arrival of the ram 31 at the lower end is detected. The opening of the contact of the lower limit switch 34 causes the controller 41 to shift the directional valve 37 for filling the lower chamber 35b of the cylinder 35 with the hydraulic oil and exhausting the upper chamber 35a. This allows the ram 31 to automatically move upward. As the ram 31 starts moving upward, the contact of the lower limit switch 34 is closed. Upon the ram 31 arriving at the upper end of its movement, the contact of the upper limit switch 33 is closed. Simultaneously, the controller 41 detects the arrival of the ram 31 at the upper limit and stops the action of the hydraulic pump 36. In this manner, an automatic return movement of the puncher is implemented.
However, said prior art has a problem that if the start switch 42 is maintained turned on, the ram 31 may automatically start again after its cycle movement. If the start switch 42 is turned on with the ram 31 staying off the upper limit of its movement, the ram 31 may move undesirably.
In common, the directional valve 37 comprises two, first and second, solenoids 37a and 37b, two pushrods 37c and 37d made of e.g. stainless steel, and a directional chamber 37e, as shown in FIG. 8. The directional chamber 37e has a spool 37f provided in its tubular oil passage for movement leftward and rightward in response to the magnetization of the first 37a and the second solenoid 37b. It is known for shifting the valve 37 upon the ram 31 arriving at the lower limit to demagnetize one 37a (or 37b) of the two solenoids and magnetize the other 37b (or 37a) at once. This however causes a remaining magnetic force of the demagnetized solenoid to activate both the left 37c and the right pushrod 37d for a brief moment thus driving the spool 37f from both sides. As the result, either the pushrod 37c or 37d may be deformed and the operating life of the directional valve 37 may be decreased.
Also, the contact of the lower limit switch 34 remains closed before the ram 31 arrives at the lower limit during the punching action. It is opened only when the punching action has been finished and then the ram 31 arrives at the lower limit. The punching in a hard material such as a stainless sheet often produces a great force of impact causing chattering or malfunction of the contact of the lower limit switch 34. If worse, the contact of the lower limit switch 34 may physically be disconnected.